This invention relates to concentrating solar energy devices, particularly those that are integrated with buildings. These devices can be thermal or electrical in principle, and they provide energy for buildings or to the electrical grid. It is advantageous that these systems are efficient and integrate readily into buildings for cost savings.
Of the currently deployed concentrating solar energy devices, the majority use relatively large optical elements consisting of parabolic mirrors or Fresnel lenses. These optics yield very high solar concentrations but with correspondingly narrow solar acceptance angles. The latter characteristic requires that the optics and solar energy receiver follow the sun by means of an accurate solar tracker. This considerable moving bulk renders these high concentration devices difficult to integrate into buildings.
In contrast, a concentrating mirror in the shape of an open-ended shallow trough with circular arc profile, commonly referred to as a cylindrical mirror, can remain fixed as part of a building roof yet provide a medium degree of solar concentration. A cylindrical mirror is often classified in the field of the invention as a non-imaging mirror. Other non-imaging mirrors have been taught in the prior art, such as those with anticlastic or dual-parabolic shapes, but their complex curvatures make them difficult to manufacture and integrate into buildings. A cylindrical mirror, however, is readily made by curving flat reflective sheets and fixing them to supports with pre-cut curvatures.
A cylindrical mirror projects an oblong area of focus parallel to its axis of curvature. As the sun traverses a cylindrical mirror that is longitudinally oriented east-west, focus changes in both position and concentration ratio. The compiled area of this dynamic focus is the cylindrical mirror's cumulative area of focus. In order to collect solar energy efficiently, a narrow linear receiver is dynamically positioned by a tracking mechanism within the cumulative area of focus.
Cylindrical mirrors and other non-imaging primary mirrors in concentrating solar energy systems typically have oblong axes of curvature oriented east-west. In this orientation, the receiver, also aligned east-west, tracks across the mirror's width as solar declination varies seasonally. The annual extent of solar declination is approximately 47 degrees at latitudes between 30 and 40 degrees north, where promising solar sites abound. This indicates that the receiver need only move tiny increment from day to day, and its position never exceeds the field of the mirror.
A cylindrical mirror's peak ratio of concentration varies inversely with its arc. For example, an arc of 72° will yield approximately 31× peak concentration, while a shallower arc of 40° will yield approximately 71× peak concentration. Collecting solar energy at higher concentration by using minors with shallow arcs is advantageous, as the minor requires less material for its construction. A cylindrical mirror with shallow arc profile also integrates more readily into a building roof, and a relatively narrower receiver can be utilized.
A cylindrical mirror in east-west orientation is ideally inclined toward the equator at an angle from the horizontal approximately equal to the latitude of the cylindrical minor's location. At this angle, solar declination during an equinox translates to an incidence normal to the cylindrical minor's chord. As solstice approaches, solar incidence gradually moves off normal. The limit, at solstice, is approximately 23° off normal for locations at latitudes between 30 and 40 degrees.
As solar incidence moves off normal, concentration ratio of a cylindrical minor decreases. For example, when solar incidence is normal to the chord of a cylindrical mirror of 40° arc, concentration ratio is 71×. When solar incidence is 20° off normal, however, concentration ratio is reduced to 48×. As such, a solar energy receiver just wide enough to encompass a cylindrical minor's focus at normal incidence would be too narrow to capture the wider focus at 20° off normal.
A cylindrical minor's variation in concentration therefore requires careful selection and optimization of a solar receiver. Highly efficient evacuated solar tube collectors are now standard components in parabolic trough concentrating devices. These devices are in widespread use for utility-scale concentrating solar power systems. Evacuated solar tube collectors are most commonly available as a 70 mm diameter absorber tube surrounded by a 120 mm evacuated clear glass tube.
The high efficiency that makes these solar tube collectors attractive for use in parabolic concentrators also benefits cylindrical mirror concentrating devices. However, when solar incidence is off normal with respect to a building-integrated cylindrical mirror of practical size, focal width will exceed the 70 mm aperture of standard collector tubes. This off-normal focal width could be captured with a double row of collector tubes but at a penalty of double expense and weight. A lower-cost solution is use of additional optics to augment solar energy collection of a single-row of collector tubes.
Another design challenge for systems with non-imaging concentrating mirrors is finding the optimum path for a receiver to track the minor's dynamic focus. Two principal tracking methods have been employed in prior art, by mounting the receiver on pivoting arms that direct it in an arc path, or by mounting the receiver to a linear tracking device that drives the receiver along a linear path.
When these paths were carefully optimized by the inventor, specifically for a cylindrical mirror with 40° arc, differences in annual energy yield was inconsequential. However, a pivoted tracker places the entire receiver weight and torsional load onto relatively small mounting areas, while a linear tracker distributes these loads across a relatively broader area. This is a concern when the mounting areas are wood or concrete block walls, since these materials are prone to fracturing under repetitive stress.
In prior art, roof-mounting of a tracking mechanism creates several problems, namely, increased roof load, increased solar shading of the primary mirror, difficult access for maintenance and monitoring, and additional roof penetrations. Mounting of a tracking mechanism on a building's sidewalls obviates these issues.
A sidewall-mounted tracking mechanism, however, necessitates a receiver long enough to span clear of the building roof. The receiver's additional thermal expansion and elasticity must be dealt with in order to reduce stress on structural components. This issue has not been adequately addressed in prior art but can be solved with appropriate structural design.